A pneumatic weighing device is the subject of this invention.
Everyday weighing devices include delicate mechanisms liable to go wrong or be damaged during use or maintenance operations, which makes them unsuitable for some situations, particularly in sealed enclosures, insulated from the outside, where only rather awkward remote operator devices are able to work: and some corrosive, humid, very hot or radiation charged atmospheres may make the operating conditions even more difficult.
This is why a weighing device has been designed which is entirely devoid of mechanisms and where only the force of expansion of a compressed gas is used to supply the energy necessary for operation and to measure the weight of the object being weighed. Such a device normally requires no on-the-spot adjustment or upkeep, and it is designed to be fully resistant to jarring caused by placing the object to be weighed. Moreover, the operation starts from an actual weight (that of the tray when empty), which allows the proper operation of the apparatus to be permanently monitored. The flow of compressed gas provides cleaning, anti-corrosion protection and cooling of the apparatus.
The device is distinguished from prior pneumatic scales, which have a conventional structure but which are fitted with a pneumatic sensor to measure the displacement of a test body under the weight of the object being weighed, since the compressed gas is only used in them for measurement. They are moreover old-style scales, the pneumatic displacement sensors having now been replaced by sensors of another kind.
The gas used for weighing is here used to produce a levitation of the tray on which the object is laid by creating an air cushion without a test body; but as such a system is particularly unstable, additional arrangements must be made to ensure that the tray is kept in balance, confronting at one and the same time its lateral displacements, its rotation and any tipping movements produced by an object throwing it off centre when being placed on it, and without ever allowing friction between the tray and the fixed base of the device, which would prejudice the accuracy of measurement.
A satisfactory pneumatic weighing device according to the most general definition of the invention includes in these conditions a base on which is placed a flanged tray surrounding the base, a system for blowing gas at constant pressure finishing in pipes passing through the base, one of the pipes, used for weighing, being vertical and emerging under the tray, others of the pipes being lateral, and emerging through chambers facing the tray flange. The gas cushions thus formed between the base and the tray by the blown gas ensure its centring relative to the base. The device also includes a measurement means of a magnitude correlated (beforehand) with the weight of an object laid on the tray.
The gas pressure prevailing in the cushion located at the end of the vertical pipe balances the weight of the tray and of the object placed upon it. This pressure is an increasing function of the weight of the whole.
The pressure is easy to measure provided a pressure inlet is installed in this pipe and a pressure sensor is installed, even in a remote place. The purpose of the lateral pipes is to centre the tray relative to the base by removing any potential for friction. The stability of the tray against tipping is better if there are N vertical pipes similar to the previous ones, preferably arranged at the apex of a regular polygon of the same centre as the tray. A pressure inlet may then be installed on each vertical pipe, and the sum of the N measurements after processing the signals gives the weight of the object.
In a first approximation, this function may be presented as follows:                     (                  M          +          m                )            ⁢      g        =          n      ⁢              xe2x80x83            ⁢      S      ⁢              xe2x80x83            ⁢                        ∑                      i            =            1                                i            =            n                          ⁢                  xe2x80x83                ⁢        Pi              ,  where
M=Tray mass
m=Object mass
n=Number of chambers
S=Surface of a chamber
Pi=Pressure prevailing in th chamber.
Precautions may also be taken against rotational movements of the tray, in the frequent situation where it is circular like the flange and the base: the base and the flange are then shaped with a vertical groove and a pin engaged in the groove, leaving two clearances in front of opposite lateral faces of the pin, and the gas blowing system also includes two lateral pipes leading respectively to clearances in opposite directions. The pin slides freely in the tray groove without contact with it. dr
A Preferred embodiment of the invention will now be described using the following figures:
FIG. 1 is a general view of the invention;
FIG. 2 is a horizontal cross-section of the upper part of the scales;
and FIG. 3 is a detail of FIG. 2.